Study on Late Cretaceous-Cenozoic exhumation of the Yanji area,NE China: insights from low-temperature thermochronology

The Yanji area,northeastern China,a part of the orogenic collage between the North China Block in the south and the Jiamusi-Khanka Massifs in the northeast,is the most likely location where the Pacific Plate subduction・related magmatic activities and subsequent exhumation processes occurred.Here,we report new low-temperature thermochronology of apatite and zircon data from the granitoid samples in the Yanji area.The exhumation rates of Tianfozhishan,Yanji area,were〜0.049 and〜0.073 mm/year,interpreted from the elevations and apatite and zircon fission track ages,respectively.The exhumation,integrated with the geological setting,suggested that the paleogeothermal gradient of the Tianfozhishan,even extending to the Yanji area,was possibly to be greater than 35℃/km in the Late Cretaceous.The thermal history modeling of the data indicates a basically similar pattern,but the various timing for different samples between the Oligocene-Early Miocene and the Middle Miocene in the Yanji area.We hence conclude that a fourstages of cooling,from〜6.7℃/Ma(during the Late Cretaceous),to〜0.8℃/Ma(during the Late Cretaceous to the Oligocene-Early Miocene),then to〜2-3℃/Ma with varied styles(between the Oligocene-Early Miocene and the Middle Miocene),and finally to<0.2℃/Ma(since the Middle Miocene),has taken place through the exhumation of the Yanji area.The maximum exhumation is>3 km under a reasonable paleogeothermal gradient(>35℃/km),speculated from the possible exhumation rate of Tianfozhishan.Combined with the tectonic setting,this exhumation,including two stages of pronounced tectonic uplift and denudation and two stages of weak exhumation driven by the low regional erosion rate,is possibly related to the subduction of the Pacific Plate beneath the Eurasian Plate since the Late Cretaceous.This study used more robust evidence to propose higher paleogeothermal gradient(>35℃/km),reflecting exhumation of>3 km in the Yanji area since the Late Cretaceous.

Generation of Cenozoic intraplate basalts in the big mantle wedge under eastern Asia

The roles of subduction of the Pacific plate and the big mantle wedge(BMW) in the evolution of east Asian continental margin have attracted lots of attention in past years. This paper reviews recent progresses regarding the composition and chemical heterogeneity of the BMW beneath eastern Asia and geochemistry of Cenozoic basalts in the region, with attempts to put forward a general model accounting for the generation of intraplate magma in a BMW system. Some key points of this review are summarized in the following.(1) Cenozoic basalts from eastern China are interpreted as a mixture of high-Si melts and low-Si melts. Wherever they are from, northeast, north or south China, Cenozoic basalts share a common low-Si basalt endmember, which is characterized by high alkali, Fe_2O_3~T and TiO_2 contents, HIMU-like trace element composition and relatively low ^(206)Pb/^(204)Pb compared to classic HIMU basalts. Their Nd-Hf isotopic compositions resemble that of Pacific Mantle domain and their source is composed of carbonated eclogites and peridotites. The high-Si basalt endmember is characterized by low alkali, Fe_2O_3~T and TiO_2 contents, Indian Mantle-type Pb-Nd-Hf isotopic compositions, and a predominant garnet pyroxenitic source. High-Si basalts show isotopic provinciality, with those from North China and South China displaying EM1-type and EM2-type components, respectively, while basalts from Northeast China containing both EM1-and EM2-type components.(2) The source of Cenozoic basalts from eastern China contains abundant recycled materials, including oceanic crust and lithospheric mantle components as well as carbonate sediments and water. According to their spatial distribution and deep seismic tomography, it is inferred that the recycled components are mostly from stagnant slabs in the mantle transition zone,whereas EM1 and EM2 components are from the shallow mantle.(3) Comparison of solidi of garnet pyroxenite, carbonated eclogite and peridotite with regional geotherm constrains the initial melting depth of high-Si and low-Si basalts at <100 km and^300 km, respectively. It is suggested that the BMW under eastern Asia is vertically heterogeneous, with the upper part containing EM1 and EM2 components and isotopically resembling the Indian mantle domain, whereas the lower part containing components derived from the Pacific mantle domain. Contents of H_2O and CO_2 decrease gradually from bottom to top of the BMW.(4) Melting of the BMW to generate Cenozoic intraplate basalts is triggered by decarbonization and dehydration of the slabs stagnated in the mantle transition zone.

Magnetotelluric data reveals subduction polarity and reactivation of the Mudanjiang suture zone,Northeast China

The Jiamusi and Songnen blocks converged in the easternmost segment of the Central Asian Orogenic Belt as a result of the subduction and subsequent closure of the Mudanjiang oceanic plate during the Permian-Jurassic.The Mudanjiang suture zone was later directly affected by subductions of the Paleo-Pacific plate and Pacific plate and is therefore an ideal place to study the subduction polarity and later transformation of a paleo-suture zone.Using three-dimensional inversion of magnetotelluric data collected along a 160-km-long profile across the Mudanjiang suture zone,we established a resistivity model of the suture zone and adjacent area.Our results reveal the subduction polarity and subduction trace of the Mudanjiang oceanic plate and provide geoelectrical evidence for reactivation of the Mudanjiang suture zone induced by the(Paleo-)Pacific plate subduction.The suture zone shows a complex conductive structure.The west-dipping crustal-scale conductor beneath the Songnen-Jiamusi collision zone represents the fossil subduction zone and indicates the westward subduction polarity of the Mudanjiang oceanic plate.Furthermore,the Mudanjiang fault identified by surface geology does not fully represent the deep structure of the Mudanjiang suture zone.The definition of the suture zone should be extended to the whole conductive region with a lateral extent of~70 km.Solid conductive minerals beneath the arc in front of the subduction zone were exhumated up from deep to the upper crust.The“chimney”-shaped conductor connected with the mantle represents the intrusive pathways of mantle-derived materials,suggesting that the Mudanjiang suture zone was reactivated by subductions of the Paleo-Pacific plate and Pacific plate,leading to remelting of the cooled and crystallized materials in the pathways.Therefore,subduction of the(Paleo-)Pacific plate destroyed the lithospheric structure of the paleo collision zone in the eastern segment of the Central Asian orogenic belt,and the large-scale crustal conductor beneath the suture zone reflects reactivation of the paleo-suture zone.